Propylene is second in importance only to ethylene as a petrochemical raw material building block. Propylene has traditionally been obtained as a by-product from steam cracking to produce ethylene and from refinery fluidized catalytic cracking processes to produce gasoline. The projected growth in demand for propylene has started to exceed that of ethylene so that existing processes cannot satisfy the foreseeable future growth in the demand for propylene.
Fluidized catalytic cracking, or FCC, is a well-known and widely practiced process for converting heavy hydrocarbons, gasoils and residues into lighter hydrocarbon fractions. The process for the catalytic cracking of heavy hydrocarbons, gasoils and residues is well known and currently practiced in all types of FCC units processing a variety of these feedstocks.
In general terms, the process for the cracking of hydrocarbon feedstocks relies on contact with fluidized catalytic particles in a reaction zone maintained at appropriate temperatures and pressures. When the heavier feed contacts the catalyst and is cracked to lighter products, carbonaceous deposits, commonly referred to as coke, form on the catalyst and deactivate it. The deactivated, or spent, catalyst is separated from the cracked products, stripped of removable hydrocarbons and passed to a regeneration vessel where the coke is burned from the catalyst in the presence of air to produce a substantially regenerated catalyst. The combustion products are removed from the vessel as flue gas. The regenerated and heated catalyst is then recycled to the FCC unit. A general description of the process as related to catalytic cracking with short duration contact times is provided in U.S. Pat. No. 3,074,878, the complete disclosure of which is incorporated herein by reference.
Various methods and apparatus have been proposed for increasing or enhancing the output of particular product streams from the FCC unit. In some cases, ancillary reactors and other treatment vessels have been provided to treat a particular fraction or reaction product stream. In some instances, multiple reactors are provided, each with a different feed, in order to derive a particularly desired product stream.
It is known from the prior art to employ a downflow reactor for processing various grades of oil, including heavy oils. It is also known to recover light olefins, e.g., ethylene, profylene and butane, and gasoline product streams from a downflow reactor along with other reaction products and unreacted feed.
A downflow reaction zone is described in U.S. Pat. No. 5,904,837 for the fluid catalytic cracking of oils, including straight-run and cracked gas oils, vacuum gas oil (VGO), atmospheric and reduced-pressure distillation residues and heavy fraction oils obtained by hydrorefining the residues and gas oils, either individually or as mixtures. The process employs a downflow type reaction zone, a separation zone, a catalyst stripping zone and a catalyst regeneration zone. The use of a temperature controlling quench oil at the outlet of the reactor is also disclosed. The principal product stream obtained was gasoline, e.g., about 38%-40% of the yield with a maximum of 16% propylene.
Another downflow FCC process is disclosed in U.S. Pat. No. 5,951,850 in which process conditions, reaction zone temperature, catalyst/oil ratios and catalyst regeneration zone temperatures are controlled to crack a variety of heavy fraction oils to provide relatively less dry gases, such as hydrogen, methane and ethane, and provide relatively higher yields of light fraction olefins. The use of more severe operating conditions, i.e., reaction temperatures and catalyst/oil ratios, produces somewhat more light olefins at the expense of reduced gasoline products in this FCC process.
Another method for operating a downflow FCC reactor for use in the processing of gas oil or heavy oil is disclosed in U.S. Pat. No. 6,656,346 and affords the recovery of significant quantities of light olefins. In this process, two types of zeolites are employed, the reaction zone temperature range is narrower than was disclosed in U.S. Pat. No. 5,951,850 and the contact time is shorter. Conversion to propylene ranged from about 20% to almost 24% by weight of the total conversion yield.
Each of the above downflow FCC unit operations includes a catalyst regeneration vessel to burn the coke from the spent catalyst and raise the temperature of the catalyst to provide heat for the endothermic cracking reaction.
The prior art relating to FCC apparatus and processes also includes examples of multiple reactor stages that are provided with different feedstocks that can be used to produce product streams containing light olefins. However, none of these disclosures provides a solution to the problem of enhancing the production of light olefins, and particularly of propylene in significant measure as an adjunct to existing FCC unit processes.
It is therefore an object of the present invention to provide a process in which a feedstream from an external source, such as heavy oil or from the same oil feedstock used in the FCC process, is further cracked to provide an enhanced light reaction product stream.
It is a further object of the invention to provide such a process that can be run efficiently utilizing the same catalyst employed in the FCC unit.
Yet another object of the invention is to provide a novel process for efficiently cracking a heavy hydrocarbon, gasoil and/or resid oil feedstock to produce a lighter hydrocarbon product stream consisting of ethylene, propylene, butylenes, and gasoline, which reaction product stream can either be recovered separately and further fractionated to recover the individual components or combined with an effluent stream from the FCC unit for further fractionation.
The term “heavy oil feed” shall be understood to include any hydrocarbon charge stock boiling in the range of 600° F. to 1050° F., or higher.